docs: documents chat message replies

[meryldakin]

Description

• Add chat reply documentation under the Chat integrations section using the current workflow-builder labels.
• Document replies to previous chat steps and existing Slack or Discord threads.
• Explain provider-specific parent message IDs, bot-based connection requirements, Slack destination fallback, and metadata.external_id usage.
• Clarify parent-step eligibility across branches and when a step produces multiple messages.
• Add a Slack trigger-data example, troubleshooting guidance, sidebar navigation, and links from related documentation.

Validation

• yarn format.check
• yarn lint
• yarn type-check
• git diff --check origin/main...HEAD

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Project	Deployment	Actions	Updated (UTC)
docs	Ready	Preview	Aug 19, 2026 6:43pm

[cursor]

Risk MEDIUM: Adds documentation for replying to Slack chat messages, including a new page, sidebar entry, and cross-links from related docs.

Reasons

• Adds a new MDX page (content/designing-workflows/replying-to-chat-messages.mdx, 100 lines).
• Updates sidebar navigation in data/sidebars/platformSidebar.ts to surface the new page.
• Changes span multiple documentation sections (designing-workflows and integrations/chat/slack).
• Content-only changes with no component, dependency, or build configuration modifications.
• Author meryldakin is a known team member; approval withheld because risk is MEDIUM, not LOW.

Notes

• Verify the new page follows Knock style guide (sentence case, "enable" vs "allow", active voice).
• Confirm cross-links resolve correctly: /designing-workflows/replying-to-chat-messages and anchor #reply-to-an-external-message.
• Review Liquid and JavaScript code examples for accuracy (parent message ID, channel ID, trigger payload).
• Check the troubleshooting table for technical correctness against Slack reply behavior.
• Confirm sidebar placement under "Designing workflows" is the intended navigation location.

  

_{Sent by Cursor Automation: Docs PR classifier}

[cellomatt]

Approved, with a bunch of mostly-small nits.

Main thing that we should change in my opinion is moving this documentation under Integrations > Chat, which will also require updating all of the links that reference the new page.